Induction hobs for preparing food are well known in prior art. Induction hobs typically comprise at least one induction heater which is associated with at least one induction coil. For heating a piece of cookware placed on the induction hob, the induction coil is coupled with electronic driving means for driving an AC current through the induction coil. Said AC current generates a time verifying magnetic field. Due to the inductive coupling between the inductor coil and the piece of cookware placed on the induction hob, the magnetic field generated by the inductor coil causes eddy currents circulating in the piece of cookware. The presence of said eddy currents generates heat within the piece of cookware due to the electrical resistance of said piece of cookware.
Typically, the electric driving means generate AC currents at frequencies outside the frequency spectrum audible for the human ear. In this way the generation of audible sounds during the operation of the induction hob is avoided. However, induction coils placed in close proximity to each other may even generate audible noise due to interference effects. If the first induction coil is driven at the first frequency and the adjacent second induction coil is driven at a second frequency, an interference frequency may be generated resulting from the difference of the first and second frequencies.
The output power of induction hops is typically changed by adapting the frequency of the AC current driven through the induction coils. Thus, the frequency difference of the AC currents of adjacent induction coils is changing according to the user's power request at the respective induction heaters.
Document EP 2 469 970 A2 discloses a cooking device with several induction heaters. The induction heaters are coupled with driving means for powering the induction heaters. In order to avoid interference noise and achieve a certain output power at the induction heaters, the first induction heater is driven by an AC current with a constant frequency wherein the second induction heater is driven by an AC current with alternating frequencies. Thereby the output power of the second induction heater is also alternating.
A drawback of the known induction hob is that interference noise is not suppressed sufficiently. In addition, the alternation of output power leads to flicker at the mains supply.